Compositions of antioxidants of reduced volatility

ABSTRACT

For incorporation in normally oxidizable polymers, polymeric antioxidants characterized by repeating hindered bisphenolic units, the average number of said units per polymeric chain in said composition being from about 2 to 10, preferably about 2 to 7, most preferably about 2 to 4. The antioxidants are prepared by oxidative coupling of appropriate bisphenols, as by reaction with a ferricyanide coupling agent. Polymer-antioxidant combinations formed according to the invention have excellent high temperature oxidative stability, high resistance to polymer damage by massive doses of ionizing radiation and the antioxidant does not release significant amounts of condensable species when subjected simultaneously to conditions of high vacuum and elevated temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of my copending application Ser. No. 481,740filed June 21, 1974, now U.S. Pat. No. 3,986,981 which is acontinuation-in-part of my copending application Ser. No. 150,831 filedJune 7, 1971 (now abandoned), which was in turn a continuation-in-partof my application Ser. No. 660,198, filed Aug. 14, 1967 (now abandoned).

BACKGROUND OF THE INVENTION

Olefin polymers are particularly subject to severe deterioration fromthe oxidative action of air at elevated temperatures. They are alsovulnerable to degradation by the action of ionizing radiation when largeamounts of such radiation are deposited into the polymer, e.g., amountsof 5 × 10¹⁰ ergs per gram and higher. Oxidative deterioration atelevated temperatures is manifested by surface crazing, then crackingand ultimately a complete embrittlement.

It is known that small amounts of some antioxidants, e.g.,4,4'-thiobis(6-5-butyl-m-cresol), can be added to polyolefins and moreparticularly to polyethylenes to prevent or impede these degradativeeffects without the contribution of color to the polymer. However, suchconventional antioxidants are not effective under conditions of highvacuum and/or high temperatures since with time the antioxidantvolatilizes away from the substance into which it was incorporated.

Until recently, the volatility of such antioxidants in a crosslinkedpolyolefin composition was undesirable primarily because of thereduction in oxidative stability which occurred as the antioxidantvolatilized away.

The use of crosslinked polyolefin materials in space vehicles hasbrought to light a still more important disadvantage of theseantioxidant materials. When used in a space vehicle, a crosslinkedpolyolefin material, employed for example in the form of electrical wireinstallation on solar panel wiring or in front of tubing, may be exposedto solar radiation in the hard vacuum of space such that the polyolefinmay be heated to temperatures as high as 125° to 150° C. Under suchconditions, most of the prior art antioxidants, such as those previouslymentioned, will be volatilized away. Since optical systems in the spacevehicle, shielded to solar radiation, may be at a relatively lowtemperature, perhaps 10° C., the gas formed from the antioxidant willcondense on the optical system thereby adversely effecting itstransmitive and reflective optical characteristics. Furthermore, theheated polyolefin with its antioxidantradiation protectant volatilizedaway is now vulnerable to radiolytic degradation such as that which maybe caused by energetic solar protons.

It is also known that certain polymeric involatile materials may be usedas antioxidants. Thus, certain phenolformaldehyde polymers may be usedas antioxidants. Also, bisphenols and triphenols such as those describedin U.S. Pat. No. 3,247,262 may be used to protect against oxidativedegradation. The bis and triphenols are still volatile, however. Thephenol-formaldehyde resins, which have lower volatility, havedisadvantages including thermosetting characteristics of thephenol-formaldehyde antioxidant, choice of precursors on the basis ofsynthetic convenience rather than antioxidant capability and apronounced tendency to impart coloration to polymers to which they areadded.

U.S. Pat. No. 3,133,899 to Kwiatek proposes the copperchloride-catalyzed oxidative polymerization of a thiobisphenol to formpolymeric ethers of formula ##STR1## the thiobisphenols employed havingthe formula ##STR2## wherein at least one R on each phenolic group isalkyl, the other being alkyl or hydrogen, alkyl in every case preferablyhaving 1 to 3 carbon atoms. Should those R groups be of more than 3carbon atoms, the work of Hay et al in J.A.C.S. 81,6335 (1959) withoxidative coupling of 2,6-disubstituted phenols would suggest that theKwiatek procedure would yield diphenoquinones as the sole product. Suchmaterials form undesirable color bodies when blended in olefinicpolymers and the like.

Braus et al, in U.S. Pat. No. 3,462,375 disclose antioxidantcompositions of formula ##STR3## but the values assigned Z₁₋₂, Y₁₋₃ andM₁₋₃ make clear that it is a phenol-substituted benzene compound ratherthan a bisphenolic polymer which is proposed. Again, in U.S. Pat. No.3,377,333 and 3,044,960, respectively, to Ciesielski et al and Morway etal alkyl-substituted thiophenolic polymers are disclosed whose repeatingunits are, e.g., ##STR4## Such polymers are distinguished from thebisphenolic polymers of the present invention by, inter alia, thecharacteristic lack in the former of direct carbon-carbon bondingbetween the phenolic rings of adjoining thiobisphenolic repeating units.

SUMMARY OF THE INVENTION

According to this invention, there are provided antioxidants andantioxidant-containing polymer compositions, the antioxidants having theformula

    H -- Z -- Y -- X).sub.n H

wherein n is from about 2 to 12 and wherein for each repeat unit, Z andX are independently selected from the group consisting of ##STR5## R₂and R₃ being independently hydrogen or alkyl, cycloalkyl, aryl oralkaryl groups having from 1 to 8 carbon atoms, R₄ being alkyl of 4 to 8carbon atoms, perferably tertiary alkyl and Y is selected from the groupconsisting of sulfur methylene and butylidene. It has been found thatthe controlled oxidative coupling reaction of certain bisphenoliccompounds will produce a non-volatile compound which is surprisinglyeffective as an antioxidant. The polymeric antioxidants of the presentinvention are generally colorless or only very slightly colored and thusmay be used at relatively high levels, i.e., 1% in polymers such aspolyolefins without causing undue coloration in the finished fabricatedarticle. The antioxidants of the invention are resistant to the effectsof high temperature and high vacuum and are capable of impartingradiation resistance to polymeric compounds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Low molecular weight phenolic compounds suitable as precursors in thisinvention have the following formulae: ##STR6## wherein R₁ is hydrogen;R₂ and R₃ are independently selected from the group consisting ofhydrogen or alkyl, cycloalkyl, aryl or alkaryl groups having from 1 to 8carbon atoms, R₄ is an alkyl group having from 4 to 8 carbon atoms and Yis selected from the group consisting of sulfur, methylene orbutylidene. R₄ is preferably tertiary alkyl, i.e., at least one carbonatom is substituted only by carbon. Phenolic moieties whose valencebonds to Y are para to hydroxyl, such as compound (2) above arepreferred from the standpoint of avoiding coloration of polymers inwhich the antioxidants of the invention are employed.

The low molecular weight bisphenolic compounds suitable for use asprecursors in this invention may be selected from a wide variety ofavailable phenolic compounds which have antioxidant properties, e.g.,4,4' methylene bis (6-t-butyl-m-cresol); 4,4' thiobis(6-t-butyl-m-cresol); 4,4' butylidene bis(6-t-butyl-m-cresol);2,2'-methylene bis (6-t-butyl-m-cresol); 2,2' thiobis(6-t-butyl-m-cresol); 2,2'-butylidene-bis(6-t-butyl-m-cresol). These andother bisphenolic precursors suitable in this invention may beconventionally prepared by, e.g., reaction of one or more phenoliccompounds with sulfur mono- or dichloride; by the reaction of olefin anda suitable thiobisphenol in the presence of an alkylation catalyst, etc.

The precursor used to produce the antioxidants of the present inventionmust have available at least one unsubstituted ortho or para positionrelative to the phenolic hydroxyl groups. For example, in4,4'-thiobis(6-t-butyl-m-cresol) there are two ortho positionsunsubstituted.

Normally, oxidative coupling yields a mixture of molecular species ofvarying length (e.g., n ranges from 2 to about 12), n average for themixture being from about 2 to about 10, preferably from about 2 to about7, and more preferably from about 2 to about 4.

The antioxidants of the present invention are useful in polymersgenerally and particularly olefin polymers and copolymers such aspolyethylene, polypropylene, polyisoprene, ethylene-propylenecopolymers, acrylic polymers such as polyethyl acrylate and otherpolymers such as polyamides, Teflon, etc., which are known to bestabilized by hindered phenols. The antioxidant can be incorporated ineffective amounts into the polymer by conventional methods such as hotmilling, mixing in a Banbury mixer, extrusion, etc. Normally, from about1 to about 10 percent by weight of antioxidant is employed, based uponthe weight of the normally oxidizable polymer.

The final polymeric systems containing these polymeric stabilizers areparticularly suitable where it is desired to crosslink by the use ofionizing radiation because these stabilizers do not appreciably inhibitcrosslinking as compared with the low molecular weight precursors andare more resistant to radiation damage.

The coupling reactions suitable for use in this invention includeoxidative coupling of the bisphenols by oxidizing agents such as K₃Fe(CN)₆, chromic acid, organic peroxides, manganese dioxide, potassiumpermanganate, potassium dichromate and electrolytic oxidation. Couplingagents which do not form molecular oxygen upon decomposition areemployed to avoid formation of undesirable by-products. Coupling maytake place in situ although it is preferred to perform the couplingreaction under conditions such that the structure and nature of theresulting antioxidant can be controlled to prevent undesirable sidereactions.

It has been found that the antioxidant polymer of this invention isparticularly effective if it is formed using a ferricyanide couplingagent, e.g., K₃ Fe(CN)₆. The increased effectiveness obtained by thismethod may be due to the fact that undesirable by-products aresubstantially reduced. Thus, other coupling systems may produceby-products which detract from the effectiveness of the antioxidant. Inany event, the antioxidants prepared with K₃ Fe(CN)₆ do exhibitincreased effectiveness when compared with antioxidants of thisinvention prepared from the same precursors with other oxidizing agents.

Any suitable ferricyanide may be used in the coupling reaction, e.g., K₃Fe(CN)₆ Na₃ Fe(CN)₆, etc. The molar ratios of ferricyanide to bisphenolmay vary from about 0.5 to 2 with the preferred ratio being about 1.33to 1.

Increasing the ratio of ferricyanide to bisphenol will increase themolecular weight of the antioxidant. At a ratio of 1.33 to 1, theresulting polymeric antioxidant will contain an average of about 3bisphenol units. The molecular weight can also be controlled bycontrolling the reaction period, a shorter reaction time resulting inlower molecular weights.

The ferricyanide may be added to the bisphenol as a water solution.Addition may be dropwise or the entire water solution may be added atonce to the bisphenol dissolved in an organic solvent.

It is important to maintain the pH of the reaction solution near 7.Thus, the pH may vary from about 5 to 9 without any substantial adverseeffect. In order to maintain the pH at the desired level, an acidacceptor or buffer such as sodium carbonate or bicarbonate can be used.In general, any acid acceptor which will maintain the pH near 7 may beused.

The reaction temperature should be maintained at as low a level aspossible. Thus, the temperature should be maintained below about 80° C.and preferably below about 50° C. If the temperature is allowed toexceed 80° C., discoloration of the polymer and decreased antioxidanteffectiveness may result.

The oxidative coupling using an organic peroxide can be accomplishedeither in solution or in situ by adding the peroxide to a polymercontaining an antioxidant precursor. Suitable peroxides include t-butylperbenzoate, dicumyl peroxide, benzoyl peroxide,2,5-dimethyl-2,5-bis(t-butylperoxy)hexane,2,5-dimethyl-2,5-di(t-butyl-peroxy)hexyne-3, t-butyl-peroxy-isopropylcarbonate, etc.

Suitable solvents in which the coupling with organic peroxide can becarried out include cyclohexane, benzene, toluene, xylene,chlorobenzene, etc. In general, the preferred solvent with peroxides isxylene, because of its lower vapor pressure at the convenientdecomposition temperature of the peroxide. Benzene is preferred when aferricyanide coupling agent is used.

The molar ratio of peroxide to the antioxidant precursor should bemaintained between a lower limit of about 0.5 and an upper limit ofabout 2 to 1 with a ratio of about 0.9 to 1 being preferred.

The present invention is further illustrated by the following examplesin which parts are by weight unless otherwise indicated.

The effectiveness of the antioxidant was measured by temperatureprogrammed differential thermal analysis in oxygen. In this technique,which has been described by V. E. Althouse (Am. Chem. Soc., Div. PolymerChem. Preprints 4, (1), 256 (1963)), the temperature of a polymer samplecontaining a certain percentage of the antioxidant is increased at auniform rate (in this case, 61/4° per minute) with a constant flow ofoxygen, until an autocatalytic oxidation (accompanied by an exotherm)occurs. The temperature at which the oxidation exotherm appears has beenshown to be proportional to the logarithm of the concentration of anantioxidant and to indicate to a degree its effectiveness as anantioxidant.

The temperature at which an oxidative exotherm occurred (hereafter knownas the DTA temperature) was measured by the above procedure on thepolymer samples as prepared, and also after the samples has beenmaintained at 200° C. for 66 hours in a slow stream of argon. it isfound that lower molecular weight antioxidants, such as 4,4'-thiobis(6-t-butyl-m-cresol), are substantially volatilized out of the polymersample after such an aging procedure. Consequently, the DTA temperatureof such samples is drastically reduced by this treatment. On the otherhand, the involatile antioxidants prepared according to the practice ofthis invention show very little change in DTA temperature after such atreatment.

Hence, the difference in DTA temperature before and after aging in argonaffords a sensitive means of measuring volatility of an antioxidant.Furthermore, because the temperature is logarithmically related toantioxidant concentration, a small difference in temperature can reflecta large difference in antioxidant concentration as shown in the tablebelow.

    ______________________________________                                        Relationship Between DTA Tem-                                                 perature and Concentration of                                                 4,4'-thiobis(6-t-butyl-m-cresol)                                                                 Approximate DTA                                            Concentration %    Temperature ° C.                                    ______________________________________                                        .01                242                                                        0.1                250                                                        0.3                268                                                        0.5                273                                                        1.0                278                                                        2.0                282                                                        ______________________________________                                    

EXAMPLES 1-2

These examples illustrate formation of my polymeric antioxidants insitu. The antioxidant was formed by milling the bisphenol and peroxideinto polyethylene and curing at 172° C. Both mixes contained 1 part perhundred of t-butyl perbenzoate which was equivalent to 1.04 mol per molof bisphenol used. Comparative runs were made using the same formulaexcept that the peroxide was omitted. The resulting compositions werecompared by a differential thermal analysis comprising heating areference and the antioxidant-containing sample at the same rate.Volatility of the antioxidant was indicated by comparing the DTA valuesbefore and after heat aging. The results are set forth in Table 1.

                                      TABLE I                                     __________________________________________________________________________                            DTA Performance ° C.                                           Color   With Peroxide                                                                          Without Peroxide                             Antioxidant     Before                                                                            After    Argon    Argon                                   Example                                                                            Type    p.h.r.                                                                           Cure                                                                              Cure                                                                              Unaged                                                                             Aged                                                                              Unaged                                                                             Aged                                    __________________________________________________________________________    1    4,4'-butylidene-                                                                      2.1                                                                              White                                                                             White                                                                             261.2                                                                              252.9                                                                             263.5                                                                              232.7                                        bis(6-t-butyl-m-                                                              cresol)                                                                  2    4,4'-thiobis(6-t-                                                                     2.0                                                                              white                                                                             Light                                                                             285.7                                                                              284.1                                                                             281.6                                                                              266.1                                        butyl-m-cresol)                                                                              Yellow                                                    __________________________________________________________________________

EXAMPLES 3-22

The method of Example 1 was repeated using various peroxides and4,4'-thiobis(6-t-butyl-m-cresol). The results are set forth in Table II.It is seen that maximum retention of performance after aging in argonresults when the weight ratio of peroxide to antioxidant precursor isbetween 1 and 2 to 1, the actual optimum value varying with the peroxideused.

                  TABLE II                                                        ______________________________________                                                                DTA                                                                           Temperatures                                          Peroxide           Cure Time at                                                                             Un-    Argon                                    Example                                                                              Type         p.h.r. 340° F. (min.)                                                                  aged Aged                                 ______________________________________                                        3      No peroxide  --     10       281.6                                                                              260.5                                4      Lupersol 101.sup.1                                                                         0.7    10       285.2                                                                              277.1                                5      Lupersol 101 .8     10       285.4                                                                              281.3                                6      Lupersol 101 .9     10       285.8                                                                              281.3                                7      Lupersol 101 1.0    10       286.1                                                                              281.6                                8      Lupersol 101 1.2    10       285.9                                                                              281.5                                9      Lupersol 101 1.4    10       285.4                                                                              282.2                                10     Lupersol 101 1.6    10       286.8                                                                              283.8                                11     Lupersol 101 0.4    10       283.5                                                                              274.7                                12     Lupersol 101 0.8    10       286.3                                                                              276.0                                13     Lupersol 101 1.6    10       285.7                                                                              281.2                                14     Lupersol 101 2.4    10       283.4                                                                              280.8                                15     Lupersol 130.sup.2                                                                         0.4    20       283.6                                                                              263.4                                16     Lupersol 130 0.8    20       284.3                                                                              278.4                                17     Lupersol 130 1.2    20       284.8                                                                              281.0                                18     Lupersol 130 1.6    20       285.3                                                                              284.1                                19     Dicumylperoxide                                                                            1.3    10       285.6                                                                              283.6                                20     Dicumylperoxide                                                                            1.5    10       285.3                                                                              283.0                                21     Dicumylperoxide                                                                            1.75   10       285.0                                                                              283.5                                22     Dicumylperoxide                                                                            2.0    10       284.0                                                                              284.3                                ______________________________________                                         .sup.1 2,5-Dimethyl-2,5-bis(t-butyl peroxy)hexane                             .sup.2 2,5-Dimethyl-2,5-di(t-butyl peroxy)hexyne-3                       

EXAMPLES 23-26

These examples illustrate preparation of the polymeric antioxidant in asolvent. A mixture of 4,4'-thiobis(6-t-butyl-m-cresol) and di-t-butylperoxide in xylene under nitrogen was heated in an autoclave and theproduct evaporated to dryness. The conditions are described in TableIII. The molecular weight of the antioxidant polymer was determined bythe freezing technique known as Beckman's method and set forth inGlasstone, Textbook of Physical Chemistry, 2 Ed (1947), 646-47.Molecular weight measurements in this experiment were made on 5%solutions in benzene.

The resulting polymeric antioxidant was incorporated into polyethyleneand the DTA exotherm temperature was determined.

                                      TABLE III                                   __________________________________________________________________________                                            DTA Exotherm                          Thiobisphenol                                                                             Concentration                                                                         Temperature                                                                          Time                                                                             Yield                                                                             Molecular                                                                           Temperature ° C                Example                                                                            kg.    gms/100 mls                                                                           ° C                                                                           Hrs.                                                                             %   Weight                                                                              Unaged                                                                             Argon Aged                       __________________________________________________________________________    23   0.108  14.5    155-160                                                                              2  105 876   287.5                                                                              283.2                            24   4.0    16.2    160    6  101 880   287.9                                                                              286.8                            25   15.0   35.8    160-167                                                                              3  101 1,287 283.5                                                                              285.1                            26   0.324  43.9    160-163                                                                              31/2                                                                             102 640   284.5                                                                              284.1                            __________________________________________________________________________

EXAMPLES 27-31

These examples illustrate the effect of pH on the polymer propertiesobtained using a ferricyanide to prepare the antioxidants. In Examples27-31, the ferricyanide was dissolved in 80% of the water used and addeddropwise over three hours to a mixture of4,4'-thiobis(6-t-butyl-m-cresol), benzene, an acid acceptor, and theremaining water. Molecular weights of the product were determinedaccording to the method of Example 23.

                                      TABLE IV                                    __________________________________________________________________________         Thiobisphenol                                                                        K.sub.3 Fe(CN).sub.6                                                                           Water                                                                             Benzene                                                                            Molecular                               Example                                                                            g.     g.     Type pH g.                                                                              mls.                                                                              mls. Weight                                                                              Appearance                        __________________________________________________________________________    27   100    90     NaOH 7-11                                                                             20                                                                              300 200  663   Light green                       28   120    146    NaOH 7-11                                                                             30                                                                              400 200  860   Green brown                       29   120    146    Na.sub.2 CO.sub.3                                                                  8-10                                                                             60                                                                              500 200  800   Deep yellow                       30   120    146    NaHCO.sub.3                                                                        6-8                                                                              42                                                                              500 200  900   Light yellow                      31    120*  146    NaHCO.sub.3                                                                        6--8                                                                             42                                                                              500 200  944   Very light yellow                 __________________________________________________________________________     *The thiobisphenol used in this example had been recrystallized from          benzene-cyclohexane.                                                     

EXAMPLES 32-39

The following examples illustrate the effect of varying reactionconditions on the molecular weight, color and DTA performance of theantioxidant. The reaction conditions set forth in Example 27 werefollowed except that in Examples 37, 38 and 39, the ferricyanidesolution was added in one batch at the beginning of the reaction. Allexperiments used analytical reagent grade benzene except 39 which used atechnical grade. Experiments 38 and 39 used technical grade ferricyanidewhile the others used analytical reagent grade ferricyanide. The molarratio of ferricyanide to thiobisphenol in the above experiments isuniformly about 1.33 to 1. The results are set forth below in Table V.

                                      TABLE V                                     __________________________________________________________________________                                                     D.T.A.                            Thiobisphenol                                                                        Ferricyanide                                                                         NaHCO.sub.3                                                                        Benzene                                                                            Water                                                                             Temp.     Molecular                                                                           Performance                  Example                                                                            g.     g.     g.   ml.  ml. ° C                                                                        Color Weight                                                                              Unaged                       __________________________________________________________________________                                                     ° C.                  32   300    365    105  650  1250                                                                              15-30                                                                             Lt. Yellow                                                                          922   289.0                        33   300    365    105  650  1250                                                                              50  Yellow                                                                              977   289.4                        34   500    608    175  1080 2218                                                                              35  Lt. Yellow                                                                          937   288.6                        35   1250   1520   437  2100 5000                                                                              35  Lt. Yellow                                                                          1000  288.5                        36   300    365    105  650  1000                                                                              25  Lt. Yellow                                                                          955   289.8                        37   1500   1824   520  2500 4500                                                                              35-50                                                                             Lt. Yellow  289.3                        38   300    365    105  650  1000                                                                              35  Lt. Yellow                                                                          924                                39   300    365    105  650  1000                                                                              35  Lt. Yellow                               __________________________________________________________________________

EXAMPLES 40-45

Polymeric antioxidants made according to the present invention wereincorporated into polyethylene at various levels irradiated to 10megarads, except the 2 p.h.r. level which was given a dose of 20megarads. The irradiated products were evaluated for heat agingcharacteristics and modulus of elasticity at a variety of temperatures.The results are shown in Table VI and Table VII. The compositions inTable VI and Table VII were prepared in slab form, except for the 2p.h.r. values which were prepared in the form of 0.125 in i.d., 0.020 inthick tubing.

The time to crack on flexing was determined by heating the samples tothe indicated temperature for the indicated time and bending themthrough 130° with cracking observed visually. The values in Table VIrepresent the average of the times to slight cracking and the times toserious cracking.

The elastic modulus was determined above the melting point of thepolymer by statically loading strips of polymer and measuring thestrain. The initial slope of the resulting stress-strain curve was takenas the elastic modulus.

These examples illustrate the marked advantage of polymeric antioxidantsat high concentrations and elevated temperatures. At lowerconcentration, oxidation occurs so rapidly that the effect ofvolatilization is masked. At lower temperatures, volatilization is not aproblem.

                                      TABLE VI                                    __________________________________________________________________________    Time to Crack on Flexing at 155° C and 175° C                                Concentration (p.h.r.)                                           Antioxidant  0.063 0.125 0.25  0.5   1.0    2.0                               Example                                                                            Type    155°                                                                      175°                                                                      155°                                                                      175°                                                                      155°                                                                      175°                                                                      155°                                                                      175°                                                                      155°                                                                       175°                                                                      200°                       __________________________________________________________________________    40   4,4'-thiobis(6-t-                                                                     71 29 144                                                                              41 271                                                                              79 623                                                                              172                                                                              >600*                                                                             320                                                                               78                                    butyl-m-cresol)                                                          41   A       71 30 154                                                                              48 293                                                                              94 752                                                                              220                                                                              >600*                                                                             490                                                                              130                               42   B       69 29 160                                                                              50 374                                                                              80 793                                                                              217                                                                              >600*                                                                             580                                                                              162                               __________________________________________________________________________     NOTE:                                                                         A was prepared from 4,4'-thiobis(6-t-butyl-m-oresol) using di-t-butyl         peroxide as shown in Example 23 while B was prepared using potassium          ferricyanide as in Example 35.                                                *These tests were abandoned after all samples at the 1% level had             withstood 600 hours at 155° C without cracking.                   

                  TABLE VII                                                       ______________________________________                                        Modulus (p.s.i) at 160° C.                                             Antioxidant    Concentration (p.h.r.)                                         Example                                                                              Type        0.0   0.063                                                                              0.125                                                                              0.25 0.5 1.0 2.0                           ______________________________________                                        43     4,4'-thiobis(6-t-                                                                         30    --   10   10    7  7   21                                   butyl-m-cresol)                                                        44     A           --    --   33   19   19  9   56                            45     B           --    --   21   15   17  9   69                            ______________________________________                                    

EXAMPLES 46-51

A further series of thiobisphenolic antioxidants were prepared as shownin Table VIII, following the procedure of Examples 27-31, supra.Molecular weight values reported are number average molecular weightsdetermined by vapor phase osmometry.

                  TABLE VIII                                                      ______________________________________                                        Antioxidants Based on 4,4'-thiobis-                                           (6-t-butyl-3-methylphenol)                                                                    4,4'-thiobis-                                                      Potassium  6-t-butyl-   Sodium   Mole-                                        Ferricyanide                                                                             B-methylphenol)                                                                            Bicarbonate                                                                            cular                                   Ex.  (g.)       (g.)         (g.)     Wt.                                     ______________________________________                                        46    906        660          246     1697                                    47    970        660         263      2393                                    48    810        660         220      1076                                    49a  2040       1500         553      1360                                    49b  2040       1500         553      1490                                    50   1930       1500         524      1189                                    51a  2170       1500         590      1450                                    51b  2170       1500         590      1869                                    ______________________________________                                    

The duplicate preparations 49a and 49b were combined together fortesting (average molecular weight 1430) and labelled 49. Similarly 51aand 51b were combined, (M.W. 1660) and labelled 51.

The antioxidants shown in Table VIII were blended into a low densitypolyethylene (DFD 6040, Union Carbide Corp.) irradiated to various doselevels and the elastic modulus (Young's modulus) at 160° C. measured.The samples irradiated to 20 mrads dose were hung in a circulating airoven at 185° C., and their room temperature elongation measured aftervarious exposure times. One sample containing tetrakis [methylene3-(3',5'-di-t-butyl-4'hydroxyphenyl) propionate] methane (Irganox 1010)was included in these tests for comparison purposes. All antioxidantswere incorporated into DFD 6040 (0.2 M.I.; 0.92 density) at the 2% w/wlevel. * 4,4'-thiobis-(6-t-butyl-3-methylphenol) is the same as4,4'-thiobis-(6-t-butyl-m-cresol).

                                      TABLE IX                                    __________________________________________________________________________                Elastic Modulus                                                                            Elongation at room temperature                                   at 160° C after                                                                     after days at 185° C                          Antioxidant                                                                          Mol. Wt.                                                                           10                                                                              20                                                                              40                                                                              60 mrads                                                                             0  8  14  17 21                                      __________________________________________________________________________    50     1190 5 13                                                                              40                                                                              63     560                                                                              290                                                                              230 105                                                                              60                                      49     1430 6 16                                                                              44                                                                              77     550                                                                              340                                                                              190 125                                                                              65                                      51     1660 5 16                                                                              47                                                                              81     540                                                                              360                                                                              180 110                                                                              30                                      47     2390 6 18                                                                              51                                                                              86     550                                                                              290                                                                              170 105                                                                              25                                      Irganox 1010                                                                         --   7 18                                                                              48                                                                              73     520                                                                              <20                                                                              <20 -- --                                      __________________________________________________________________________

EXAMPLES 52-57

Other oligomeric antioxidants were prepared from4,4'-butylidene-bis(6-t-butyl-3-methylphenol) as in the previousexample. Table X shows the recipes used in the preparation and themolecular weights of the products, determined as in Examples 46-51.Preparations were based on 1500 g. of the antioxidant starting material.4700 ml. of deaerated water was used in each reaction to dissolve thesalts. The compound 4,4'-butylidene-bis-(6-t-butyl-3-methylphenol) isthe same as 4,4'-butylidene-bis-(6-t-butyl-m-cresol).

                  TABLE X                                                         ______________________________________                                                Potassium  Sodium              Mole-                                          Ferricyanide                                                                             Bicarbonate         cular                                  Sample No.                                                                            (g.)       (g.)       Benzene (ml.)                                                                          Wt.                                    ______________________________________                                        52      1840       500        5400     1230                                   53      2040       553        5400     1790                                   54      1940       500        5400     1460                                   55      1840       500        5400     1240                                   56      1650       500        6500     1340                                   57      1750       500        6500     1610                                   ______________________________________                                    

Certain of the foregoing antioxidants were milled into polypropylene(Profax 6823; flow rate (g./10 min.), 0.4 (ASTM-D1238L); 0.903 density)and aged at 150° C. in a air circulation oven. The results are given inTable XI. Some of these materials were also blended with distearylpentaerylthritol diphosphite and blended into polyethylene containing apolyfunctional monomer (triallyl isocyanurate). The irradiated sampleswere aged at 175° C. and the results are shown in Table XII.

                  TABLE XI                                                        ______________________________________                                                 1      2      3    4    5    6    7    8                             ______________________________________                                        Profax 6823                                                                            99.7   99.7   99.7 99.7 99.7 99.7 99.7 99.7                          Irganox 1010                                                                            0.3   0.075  --   --   --   --   --   --                            Dilauryl                                                                      Thio-                                                                         dipropionate                                                                           --     0.225  --   --   --   0.225                                                                              0.225                                                                              0.225                         Sample 52                                                                              --     --      0.3 --   --   0.075                                                                              --   --                            Sample 53                                                                              --     --     --    0.3 --   --   0.075                                                                              --                            Sample 55                                                                              --     --     --   --    0.3 --   --   0.075                                Average ultimate elongation (at 2"/min.)                                      following heat aging at 150° C, %                               Original 500-530%                                                             4 weeks  267    355    450  320  210  --   210  --                            5 weeks  225    333    367  330  --   450  170  --                            6 weeks  180    320    350  --   --   390  --   290                           7 weeks  --     250    --   --   --   243  100  300                           ______________________________________                                    

                  TABLE XII                                                       ______________________________________                                        Samples irradiated to 25 mrads                                                             1      2      3       4    5                                     ______________________________________                                        DFD 6040       95       95     95    95   95                                  Triallyl Isocyanurate                                                                        3        3      3     3    3                                   Irganox 1010   2        --     --    --   --                                  Distearyl pentaerythritol                                                     diphosphite    --       1      1     1    1                                   Sample 52      --       --     1     --   --                                  Sample 53      --       --     --    1    --                                  Sample 55      --       --     --    1                                        M100 at 160° C, psi                                                                   91       36.5   83    85   86                                  Average elongation (at 2"/min.) after heat aging at 175° C, %          Original  >500%                                                               1 day          310      447    333   253  310                                 2 days         277      420    320   177  282                                 4 days         140-280  367    283   <20  270                                 7 days         <20      <20    30-300                                                                              <20  293                                 ______________________________________                                    

The above examples are not to be construed as limiting but merelyillustrative of the scope of the present invention. Thus, it can be seenthat the antioxidants of the present invention can be formed from a widevariety of bisphenol materials. The antioxidants are generally effectivein all polymers, particularly those which are known to be stabilized byhindered phenols, and they can be incorporated in such polymers byconventional methods such as hot milling, mixing in a Banbury mixer,extrusion, etc.

I claim:
 1. An antioxidant composition comprising a mixture ofbisphenolic polymers of formula

    H--Z--Y--X].sub.n H

wherein n is an integer of from 2 to about 12 and average n for themixture is from about 2 to about 10 and wherein Y is independentlychosen from the groups consisting of sulfur, methylene and butylideneand for each bisphenolic repeating unit --Z--Y--X--, Z and X areindependently selected from the group consisting of ##STR7## R₂ and R₃being independently selected from the group consisting of hydrogen andalkyl, cycloaklyl, aryl and alkaryl groups having from 1 to 8 carbonatoms, R₄ being an alkyl group of from 4 to 8 carbon atoms.
 2. Anantioxidant composition according to claim 1 wherein average n is fromabout 2 to about
 7. 3. An antioxidant comprising a mixture ofbisphenolic polymers having the formula ##STR8## wherein Y is selectedfrom the group consisting of sulfur and butylidene and wherein n is aninteger of from 2 to about
 12. 4. An antioxidant composition accordingto claim 3 wherein average n is from about 2 to about
 10. 5. Anantioxidant composition according to claim 4 wherein average n is fromabout 2 to about
 7. 6. An antioxidant composition according to claim 5wherein Y is sulfur.
 7. An antioxidant composition according to claim 5wherein Y is butylidene.
 8. An antioxidant composition according toclaim 3 wherein average n is from about 2 to about
 4. 9. An antioxidantcomposition according to claim 8 wherein Y is sulfur.
 10. An antioxidantcomposition according to claim 8 wherein Y is butylidene.